Thermal relay



May 23, 1933. G, w. OKEEFFE THERMAL RELAY 2 Sheets-Sheet l Filed June11, 1931 ryz.

I qg May 3 1933- 5v w. OKEEFFE THERMAL RELAY Filed June 11, 1931 2Sheets-Sheet 2 fiver-12 071 Patented May 23, 1933 UNITED STATES PATENTOFFICE GEOBlGE w. OIIEFI'R, 01' HILTON, IABSACKUBITTS, ASSIGIOB TO 001mmELECTRICAL IANUI'AUI'UBING CORPORATION, OF SOUTH BOSTON, USSAOHUBBTTS, ACORPO- RATION OF IASSACHUSETTS THEBIAL BELLY Application Medium: 11,

This invention relates to overload protective thermal relays of the typehaving a thermally-expansible element which is responsive to and isheated by current in a power circuit and which operates to controlcontact members included in a control circuit of a switch for the powercircuit, as the circuit of an electric motor. The relay, however, is notlimited to such use.

One of the objects of the present invention is the provision of animproved thermal relay of this type having means for efl'ectlng apositive and rapid separation of its contact members and specifically toprovide a thermally responsive relay having a normally stressedoperating'mem'ber and a thermally expansible element for normally holdinsaid member in an inoperative dposition an for releasing said member sudenl upon a predetermined heated condition 0 the expansible element toactuate the relay contacts with a snap action.

In thermal relays employin the usual bimetal expansible strip, the stripis frequently under an initial stress or, if free from such stress, isbent between its ends, either into U-shape, spiral or otherconfiguration. The temperature of the strip modifies its stress and itsconfiguration. The temperatures of the room in which the relays areconstructed vary from day to day. Thus, while all relays may be adaptedto operate at the same current rating, those adjusted under a certainfactory room temperature will, in fact, be different in adjustment fromthose regulated under a diflerent room temperature, and will performdifferently when in service. Hence a further object of the invention isthe provision of a relay that is practically free from the aboveobjection.

In carrying out this object I provide a bimetalstrip that is straightand free from any reflexd condition and is normally under no initialstress between rigid end abutments so that'fupon being heated, it iscaused to bend or buckle in the middle, thereby to release, like adetent, a spring actuated circuit controlling member. Such aconstruction constitutes a further object of the invention.

Another object is the provision of im- 1aa1. semi so. use.

proved means by which a common circuitcontroller is operated se aratelyby a plurality of such thermal re ya and wherein an operated rela can bereset by a part of the circu t-contro ler without modi the condition ofthe circuit as determined y the operation of the relay.

A yet further object of the invention is the provision of an improvedthermal relay havmg an electric heating element and athermally-expansible element which is located alongside and closelyadjacent said heating element and is arran when heated to flex 1n thedirection of said heating element.

A yet further object of the .invention is the PIOVISIOII' of an improvedform of thermal relay having a 'base portion which supports athermally-expansible element and a detachable cover portion whichcarries the heating element.

Another object of the invention is the rovision of a heater element fora therma rela having a grid-shaped heating portion w ich is desi ed toprevent warpin or sagging of the e ement when it is hea A still furtherobject of the invention is generally to improve the construction andoperation of thermal relays.

Fig. 1 is a front rspecti've view of the thermal relay em ying thepresent invenion.

Fig. 2 is a front elevation of the relay of Fig. 1 with the coversremoved, the position of the heating elements being indicated by dottedlines.

Fig. 3 is a section on line 3-3 of Fig. 2.

Fig. 4 is a section on line 44 of Fig. 2.

Fig. 5 is a section on line 5-5 of Fig. 2.

Fig. 6 is a pers ctive view of the supporting frame for a t ermalelement.

Fig. 7 is a view similar to Fig. 6 with the spring pressed operatingmember in place.

Fig. 8 is a detall perspective view showing a thermally-expanslbleelement.

Fig. 9 is a sectional view illustrating the position of the relay partsafter one therinzllly responsive umt has operated on over- -Fi 10 is arear perspective view of a cover showing the heating element carriedthereby.

ch constitutes an insulating support for the main andauxiliary terminalsof the relay, together with two separate andidenticalthermally-responsive relay units 12 and 14, and the cooperatinstationary and 15 movable contact members the relay (Fig. 2). The baseplate is formed with rear oppositely outstandin side flanges havingpassages 22 theret roughto receive bolts by which the relay is adaptedto be secured any desired ap aratus. The relay herein shown is espec1a ydesigned to be secured to the frame of an enclosed electromagneticallyoperated switch of the commonly employed for controlling sma power-cir-25 cuits, although it is not limited to such use.

The base 16 is provided with relatively dee transverse grooves 24 (Figs.2 and 3) whic are located adjacent the flanged ends thereof and whichextend from top to'bottom of the base. The base is further rovided withshallow conductorreceiving c annels 26 and 28 in the top and upper rearfa'ces thereof respectivelywhich are extensions of and are in alignmentwith said grooves Each of said ooves 24 contains an upper and a lowermain terminal member 30 and 32 for a separate phase conductor of theower line with which the relay is associated. gaid upper terminal member30, as shown most clearly in F' '3, comprises a flat bar or strip ofconducting material of generally U-shape comprising the spacedupstanding arms 34and 36 and the lower connectin portion 38. The arm 36is' adapted to lie in arm' 34 is located flush with the front face ofthe base. The foremost arm 34 of said terminal is provided with anintegral portion 40 which extends rearwardly in the channel 26 and witha downwardly extended integral portion 42 which lies in the conductorreceiving channel 28 on the rear face of the base. The strips comprisingsaid terminal members, as shown most clearly in Fig. 2, have a widthcommensurate with the width of the grooves 24 and the conductor passages26 and 28, whereby to completely occupy the ends of the grooves. Bolts44 extend through the rear portions 42 and are screwthreaded into thearms 36 of. said terminal members whereby to secure them fixedly to thebase. Said terminal portions 42 and bolts 44 constitute the upper mainline terminals of the relay. Said lower contact members 32compfieferably a material that can be I 56, Fig. 6, consisting of a hasthe bottom of the groove 24 and the connect transverse rectangular wells54 which are disposed between and adjacent the grooves 24 and each ofwhich 18 in communication throughout its length with one of saidgrooves. Each of said wells 54 contains a thermalllg-responsive unitwhich is adapted to be uenced by the current flowing in the phaseconductor located in the groove adjacent thereto. Saidthermally-responsive umts are identical and interchangeable. Each unitconsists of a stamped rigid frame late 58 which is disposed in' thebottom of e well, a left hand upstanding reflexed plate-stiffeningflange 60 provided with an enlarged ear 62 at the lower end thereof, anda pair of upper and lower right hand side ears 64 and 66 all of whichears are reflexed into a position normal to thebase plate 58. Thediagonally opposite ears 62 and 64 are provided with.

mutuall 'inwardly directed reflexed wings 68 and 0 each of which isprovided with a pair of aligned marginal notches 72 which are disposedin a common vertical lane and which are adapted to support a termallyexgansible strip hereinafter to be described.

aid frame also carries a pivotally' supported operating orcircuit-controlling arm 4, Fig. 7, which lies alon side the right handside of the frame. S ai "operating arm 74 is pivotally supported at oneend on a pin g 76, which extends between and is fixed in the parallelears 62 and 66 of the frame, and has its other and free end locatedintermediate the ends of the frame. Said arm is provided with a U-shapedbent end portion the arallel arms of which are closel received tween thecars 62 and 66 and thus prevent lateral displacement of the arm on itspivot pin 76. A coil spring is provided on said pin between the U-shapedarms of said member and has oppositely extended ends which en age thebase plate 58 and the U-shaped en of the arm 7 4 respectively whereby tobias the arm constantly for movement at its free end in a direction awayfrom the base plate 58. The base plate 58 of said frame is provided witha cutawa portion from the margin of which a re exed ear 86 upstandswhich is disposed closely adjacent the left hand side of the member 7 4and constitutes means to support the free end thereof against lateraldisplacement in one direction. A second upstanding ear 88 is providedclosely adjacent thread-free end portion which is adapte to be normallydisposed beneath the vertical plane including the notches 72 in whichthe thermal element is located. Said operating member 74 is furtherprovided with an outstanding arm 96 at the free end thereof whichextends a substantial distance beyond the right hand end of cars 64 and66 of the frame and constitutes a circuit-interrupting andrelay-resetting member.

A composite thermal strip 98, Fig. 8, is adapted to be supported betweenthe wings 68 and 70 of the frame and in the notches 72 and comprises afiat strip composed of two metals having different characteristics of.

thermal expansion secured back to back. Said strip is provided withlongitudinally extended pairs of projections 100 at its opposite endswhich are located in the notches 72 of the frame, whereby to support theexpansible element at its opposite ends against lateral displacement inall directions while permitting longitudinal movement of the endsthereof due to changes in length of the strip during expansion andcontraction. The projections 100 are intended to be long enough so thatthey will not come out of the notches 72 on any usual flexing of thestraight thermal element. The projections 100 and notches 72 are alsosuch as to ermit the strip to bow freely, when heated, without imposingrestraint on the ends of the termal element. The thermal element thus,in effect, floats between the wings 68 and 70, which support the thermalelement against undue longitudinal movement with respect to the latchpin-92, of the circuit-controlling arm 74.

With this construction the thermal ele ment need have no reflexedsections and is free from restraint against bowing due to .temperaturewhnges, so that the variations in room temperatures at which variousthermal elements are constructed, assembled and tested have negligibleeffect in their ultimate operating characteristics. The floating thermalelement also obviates the necessity for extreme accuracy in manufactureof the length of the element, or the distance between the wings 68 and70.

The frames 56 of the thermal units are secured rigidly in the wells 54by means of attaching screws 101, the heads of which are located in abroad recess 102 in the rear face of the base plate and two of whichextend through the base 10 and are screw-threaded into passa es 103 inthe base plate 58 of each frame. T e thermally-responsive units arelocated in the wells 54 in longitudinally transposed position so thatthe laterally extended arms 96 of the operating members 74 thereof aremutually inwardl directed toward the center of the base y thus reversingthe position of the units they can be identical in I construction.

Means are provided separately to connect electrically each pair of u perand lower terminals 30 and 32, whic means also constitutes the heaterelement for the adjacent thermally-responsive unit. As shown in Fi 3 and10, a cover 104 is provided for eac of the adjacent intercommunicatinggrooves and wells 24 and 54, on the back face of which cover is carrieda heater element 106. Said element 106 is secured permanently to thecover by means of rivets 108 located adjacent the ends thereof and isformed from a flat strip of resistance mate rial having oppositelydirected, deep marginal slots which are located on alternate sides ofthe strip and in longitudinally staggered relation whereby to provide aheatin grid having a relatively long sinuous patE of current travel fromend to end of the strip. The strip is provided with upwardly ofiset flatend portions 11(/ which are located at one side. of the lon tudinalmedian line through the strip an which when the cover is disposed incovering relation with the channel and well of the base, are disposedover and in contact with the terminal portions .34 and 46 of the upperand lower terminals 30 and 32 and consequently connect said terminalselectrically through the heater element. Attaching screws 112 extendthrough the cover and the terminals 110 of the heater element and arescrew-threaded into the foremost arms 34 and 46 of the upper and lowerterminals respectively, the

screws thus serving not only to hold the 'cover a in place but also tomaintain the heater element in good electrical contact with the terminalmembers. As shown most clearly in Fig. 10, the grid portion of theheater element is composed of two angularly-related sections one ofwhich is parallel and the other of which is normal to the cover and isadapted to lie in the base well closely adjacent the thermal element,when the cover is in osition over the well. The angularly-reated gridsections serve to strengthen the grid mechanically and to make itsufiiciently ri 'd to avoid sagging or deflectin laterally wl bn hot.The covers are provi ed'at one side of the heater element and beneaththe heating portion thereof with a raised portion or boss 114 which isadapted to be received within and to conform closely with the margins ofthe wells 54 and that portion of the grooves 24 which is enclosed by theterminals 30 and 32 thus to assist in aligning the screws spaced backfrom the line terminals 112 with the threaded passages in the contactvents the attachment of the cover in an inverte position.

An intermediate terminal 116 is provided at the lower edge of the basebetween and 32 and has aconducting portion which extends up; wardly inthe recess 102 at the back of t base and terminates in an upper maincontact member, not shown.

The stationary and movable contact members of the circuit-controller aresupported on the face of said baseplate between the thermally-responsivedevices above scribed. As shown in Figs. 2 and 4, said movable contactmembers comprise a pair of parallel spaced resilient fingers 118 and.119 which are fixed at their lower ends in rectangular channels 120 inthe face of the H base by means of bolts 122 and 124 which constitutethe auxiliary circuit terminals for the relay. 7

The stationary contact member comprises a resilient bifurcated plate 126which is secured rigidly at its upper margin to the base 10 by means ofattac ing means 128. Said plate has the bifurcations thereof extendedbeneath the ends of the contact fingers 118, 119, and provided withupturned contact flanges 130 and 131 respectively at their lower edgeswhich normally are engaged by the contact fingers 118, 119. The baseplate 10 is provided with a central recess 132 Fig. 4, located beneaththe stationary and movable contact members which is further providedwith a deeper central arcuate recess 134 in the vicinity of the mutuallyinwardly directed operating arms 96 of the thermal units. A transversebarrier 136. extends across the recess 132 beneath the movable contactfingers 118, 119 and limits the downward movement of said contactmembers upon the depression of the stationary contact member 126. Atransverse slot 138 is provided in the dividing walls 140 which separatethe. recess 132 from the wells 54, thus to permitvertical movement ofthe extensions 96 of the operating members 74. The terminals 122 and 124of the relay are intended to be connected in series in the pilot circuitof an electromagnetically operated switch, or the like, and consequentlythe elevation of either movable contact finger 118 or 119 from engaement with the stationary contact memfier 126 will effect the opening ofthe pilot circuit.

As shown most clearly in Figs. 4 and 11, an insulating member 142 iscarried by the stationary contact member 126 between the bifurcationsthereof. Said insulating member has an angularly-related rearwardlydirected extension which projects through a rectangular passage 146 insaid contact member located in the median line thereof and an upwardlyextended angularl -related toe members. The raised portion 114 alsoprertion 148 which lies paralle with and is isposed beneath the contactplate 126. Said insulating member is also provided with opitely extendedtrunnions 150, also of insuating material, which overlie and are spacedabove the 0 rating arms 96 and are directly beneath an in contact-withthe movable con- :tact fingers 118, 119, as shown most clearly in Fig.4. It will thusbe evident that the insulating member 142. will be heldin the above described position by the-contact fingers 118, 119 and the?toe portion 148. Said d member constitutes means by which the flexibleplate 126 can be flexed manually out of engagement with the contactfingers 118, 119 whereby .to break theauxiliary circuit inde- 'pendentlyof the thermal devices; and also to reset the devices after an operationthereof. The operation of the relay is as follows: If an overloadcondition occurs in the main circuit to be protected which includes inseries therewith the heater elements 106,

'suflicient heat will be generated in one or .more of said heaterelements to cause the bimetallic strip 98 adjacent thereto to flex outof its normally straight condition and to assume an arcuate position inwhich its middle point is displaced laterally and passes from beneaththe stud 94 of the actuating member 74, thus releasing the actuatingmemher which under the biasof the spring 80 flies upwardly intoengagement with the abutment '90.. In this position of the operatingarmthe extension 96 thereof has engaged a trunnion 150 of the insulatingmember 142 and has raised the member and the contact finger above itinto an elevated position in which it is free from its coo ratingstationary contact flange, thereby reaking the auxiliary circuit. Itwill be noted that the extension 96 of the operating arm strikes theinsulating member 142 a hammer blow due to its normally s aced relationtherewith and consequently e ects a rapid separation of the contacts. InFig. 9 the insulating member 142 is shown in the position it occupiesafter one of thethermally-responsive units has operated.

It will be evident that the operation of either thermally responsiveunit, since they control contacts which are in series, will effect theopening of the pilot, or auxiliary, circuit.

The resetting of the relay is accomplished by depressing the insulatingmember 142 upon which the stationary contact plate 126 is adapted to beflexed downwardly sufliciently to permit the trunnions 150 to depressthe extensions 96 of the operating arm 74 which has operated into aposition in which its stud 94 is beneath the thermal element. In orderto increase the flexibility of the stationary contact plate 129,V-shaped marginal notches 1: ment-and does not 0 and 154 are provided inthe side and top of said contact member.

, g the. resetting operation the plate 126,is moved awa from the contactfingers 118,, 1-19 so that e auxiliary circuit is not completed untilafter the relay has been reset and the plate allowed to turn to itsnormal podtion. Thus the mam circuit is alwaysprotected by a relaywhenever the auxiliary circuit is complete, which is a desirable"feature.

It be noted that the thermal element 98=has edgewise engagement with thepin 92. b

1. that the relay might equally well have normally'open contacts or thatin lace of the contact members any of the usua mechanical trippingdevices might be employed.

Various changes in the construction and arrangement of the parts may bemade without departing from the scope of the tion.

I claim:

1. A thermal relay comprising'an insulating supporting base, pairs ofinsulated main and auxiliary terminal members carried by said base,cooperating stationary and movable contact members normally electricallyconnecting said auxiliary terminal members, an operating member havingmeans which constantly biases it into a position to separate saidcontact members, thermlly-actuated means controlling the release of saidoperating member, a heater element located in heat imparting relation tosaid thermallyactuated member which is electrically connected betweensaid main terminal members, and manual means for resetting saidoperating member first and thereafter effecting the reengagement of saidcontact members.

2. A thermal relay comprising an insulating sup orting base, two pairsof main line termina s carried by and spaced apart on said base,auxiliary terminals carried by said base, cooperating stationary andmovable contact members normally electrically connecting said auxiliaryterminals, heater elements electrically connecting the pairs of mainline terminals, a thermally-responsive member located adjacent each ofsaid heater elements, operating members associated with each of saidthermally-expansible members having means normally biasing them into aposition to separate said contact members, means associated with saidthermally-ex-l invenpansible members arranged to govern the release ofsaid operating members, and manually operative means for conjointlyresetting said operating members and thereafter comple the electricalconnection between said. a 'ary terminals at said contact members. 1

3. A thermal relay having an insulating supporting base, spaced main andauxiliary terminal members carried by said basal cooperating stationaryand movable con'tact members mounted on said base and electricallyconnected with said auxiliaryterminal members, a thermally-responsiveunit located ad; acent said contact in hers comprising a spr1ng pressedoperating member which is members, a thermally-actuated latch member forwithholding said operating memberfrom operation, and a' cover memberhaving a heater element carried thereby, said heater element havingterminals that are spaced apart the same distance as said main terminalmembers, said covenmember having provision for securing it to said basewith the terminals of said heater element electrically connecting apairof said main line terminal members and also-with said heater elementdisposed in operative relation with said thermally-responsive unit.

4. In a thermal relay, a frame having opposed reflexed end portions, aflat normally used into a position to control said contact straightbimetallic strip extended between and having its ends loosely connectedwith said end portions so that said strip is free to flex throughout itslength between its ends,

a pivoted operating arm carried by said frame, means normally biasingsaid operating arm into an operating position, said arm having anabutment which is located under and normally engages a narrow face ofsaid bimetallic strip whereby to hold said operating member releasablyagainst operation, and heating means for said bimetallic strip.

5. In a thermal relay, a frame including a back portion havingupstanding end por tions, a flat bimetallic strip extended between andloosely supported by said end portions, an operating arm disposedalongside said bimetallic strip between its ends and having a pivotedsupport at one end on an end portion of said frame, means constantlybiasing said arm into an operated position, said arm having a latchportion thereof normally disposed beneath and engaged with a narrow faceof said bimetallic strip by which said arm is held from operation, andmeans for heating said strip to move the narrow face thereof out ofengagement with said latch portion.

6. In a thermal relay, a frame having a back portion, and reflexedupstanding end portions, a normally straight flat bimetallic stripcarried by said frame in a plane normal to the base having its oppositeends loosely connected with said end portions to permit lateral movementof said strip intermediate its ends, an operating arm pivoted at one endto said frame and disposed in a plane parallel with and closely adjacentsa id strip, means normally biasing said arm into an operated position,said arm having a lateral extension which is normally disposed beneaththe narrow edge of said strip intermediate the ends thereof whereby saidstrip normally, holds said arm in aninoperative position against itsbias, and a heater element disposed alongside said strip.

7. A thermally-responsive unit for a ther mal relay comprising a framehaving a back supporting portion and upstanding reflexed end portions, anormally straight, fiat bimetallic strip carried by said frame havingits ends loosely connected with said end portions whereby to permit saidstrip to flex laterally intermediate its ends while maintaining its endsfixed against deleterious lateral displacement, an operating arm havinga pivotal connection at one end of said frame extended alongside saidstrip and having its free end located intermediate the length of saidstrip, means constantly biasing said arm into an operating position, alatch portion carried by the free end of said arm arranged normally toengage said strip whereby to hold said arm releasably .by said strip inits unoperated position, and upstanding projections carried by saidframe on opposite sides of said arm constituting means to restrain saidarm from lateral movement in either direction,

8. A thermal-responsive unit for a thermal relay comprising a supportinframe having a back attachin port1on and iaving reflexed upstanding emfportions, an operating arm having a ivoted support at one end on saidframe and having means which constantly biases it toward an operatedosition, means for restraining the free end of said arm from lateralmovement in both directions comprising upstanding projections of saidframe located on opposite sides of and closely adjlacent said arm, andmeans for releasably olding said arm in an unoperated positioncomprising a normally straight bimetallic strip having its opposite endsloosely supported in the end portions of said frame and normallyoverlying an extended portion of said 0 erating member.

9. if thermally-responsive unit for a thermal relay comprising a framehaving a back supporting portion and spaced upstanding and confrontinend portions, an o erating member carried by said frame which isnormally biased into an operated position, and means normally holdingsaid arm in an unoperated position comprising a normally strai htbimetallic strip having its ends loose y supported in the end portionsof said frame.

10. A thermally-responsive unit for a thermal relay comprising a framehaving a back supporting portion and having opposed upstanding ears atone end thereof, an operating arm carried by said frame having axU-shaped end which 18 closelyreceived between the earsof said frame'and ispivotally sup-'- ported therebetween, means to guide the free end ofsaid arm against lateral displacement in bothdirections comprisingupstandin ears of said frame which engage the free en of said arm onopposite sides thereof,a spring constantly biasing said arm into an 0rated position, an adjustable latch projection carried bythe free end ofsaid arm, and means normally engaging said projection whereby to holdsaid arm in an unoperated position comprisin a normally straightbimetallic strip whic is supported at its ends on said frame and isextended alongside said arm.

11. A thermally-responsive unit for a thermal relay comprising a framehaving a back supporting portion and having upstanding re exed andconfronting cars at its 0 posite ends, an operating arm having one endthereof pivotally supported on said frame and having its free endlocated intermediate the ends of said frame, an adjustable latch portioncarried by the free end of said arm, means normally biasing said arminto an operated position, and means for holding said arm releasably inan unoperated position comprising a normally straight strip ofbimetallic material having the opposite ends thereof loosely supportedin the confronting ears of said frame, the ends of said strip havingangular projections which are received in corresponding angular passagesin said can whereby to permit free longitudinal movement of the stripupon expansion thereof while maintaining the ends thereof fixed againstlateral movement relative to the cars.

12. A" thermal relay having an insulatin supportin base provided with apair 0 spaced we ls and having a transverse channel connecting saidwells, a stationary contact member located on one side of said channelbetween said wells, a air of movable contact members fixed to sai baseon the other side of said channel and extended thereacross into aposition to engage said stationary contact member, said stationarycontact member constituting a bridging member for said movable contactmembers, a thermally-responsive unit located in each of said wellscomprising a frame having a spring pressed operating member and athermally-expansible member which normally holds said operating memberreleasably in an unoperated position, said operating members havingmutually inwardly directed extensions located in said channel eachbeneath a movable contact member, and heater elements disposed in saidwells alongside said members.

13. A thermal relay having an insulating supporting base provided with apair of spaced wells and having a transverse channel connecting saidwells, a stationary con- .tact member located onone side of said channelbetween said wells, a pair of movable member for said movable contactmembers;

a thermally-responsive unit located in each of said wells comprising aframe having a spring pressed operating member and athermally-expansible member which normally holds said 0 ratin memberreleasably in an unopera bers having mutually inward y directedextensions located in said channel each beneath a movable contactmember, and cover members for said wells having heater elements carriedthereby, and means to secure said cover members to said base with theheater elements thereof located in said wells.

14. A thermal relay having an insulating supportingl base rovided with apair of spaced w s and avin a transverse channel connecting said we ls,a stationary contact member located on one side of said channel betweensaid wells, a pair of movable beneath a movable contact member, heatingmeans for said thermally-expansible members, and manually operativemeans arranged to engage the extensions of said operatlng members toreset said members to an unoperated position.

15. A thermal rela having an insulating supporting base provlded withspaced wells therein and having a transverse channel connecting saidwells, a stationary contact member located on said base between saidwells at one side ofsaid channel, a pair of movable contact memberssecured to said base on the opposite side of said channel and extendedacross said channel into a position to enga e said stationary contactmember, a thermafiy-responsive unit located in each of said wells,comprising a support'for a spring pressed operating member and having athermally-expansive stri which constitutes means for normally hol ingsaid operating member in an unoperated position, said operating membershaving mutually inwardly directed extensions which are disposed inposition, said operating memsaid channel each beneath and in position tocontrol a movable contact member, a manual resetting member supportedabove said 'base which overlies the extensionsof said operating memberswhereby said members can be manually reset, and cover meansformingclosures for the wells in said base havm heater elements supported onthe rear ace thereof, and means to secure said covers releasably to thebase, said heater elements having rearwardly extended rejections thereofwhich enter into said we s and are disposed alongside saidthermally-expansible elements.

16. A thermal relay comprising a supporting structure, a bimetal elementhaving va loose connection with said structure by which it can float insaid structure and'bow freely in the middle when heated, means forheatmg said element so that it can bow, and means controlled by thebowing-of said element. Y I

17.. A thermal relay comprising a bimetal strip, a supporting structuretherefor having means which supports both ends of the strip looselyagainst lateral displacement but permits longitudinal movement, so thatthe strip can bow freely in the middle when heated, means for heatingthe strip, and means controlled b the bowing of the stri 18. A thermalrelay comprising a straig t bimetal .strip that is free from intentionalinitial stress, a supporting structure therefor having end members thatengage the ends of the strip loosely so that the strip can movelengthwise therein and bow therebetween when heated while restrainingthe ends of the strip against undue sidewise movement, means for heatingthe strip, and means controlled by the bowing of the strip.

19. A thermal relay comprising a straight bimetal strip that is freefrom intentional initial stress, a supporting structure therefor havingend members that engage the ends of the strip to hold it located butwithout subjecting the strip to longitudinal pressure, means for heatingthe strip so that it can bow between its ends, and means controlled bythe bowing of the strip. a

20. A thermal relay comprising a flat straight bimetal-strip that isfree from intentional initial stress and is adapted to bow when heated,a supporting structure therefor having end-supports which loosely engageopposite flat faces of the strip at the ends thereof to restrain thestrip against lateral displacement while permitting it to bow freely inthe middle when heated without exerting longitudinal thrust on thestrip, and means controlled by the bowing of the strip.

21. A thermal relay comprising a flat straight bimetal strip that isfree from intentional initial stress and is adapted to bow when heated,a supporting structure therefor having end-supports which loosely engageopposite flat faces of the strip at the ends thereof to restrain thestrip against lateral displacement whilepermitting it to bow freely inthe middle when heated without exerting longitudinal thrust on thestrip, a pivoted arm having an extension which en ges an edge of thecool strip at the middle t ereof, the strip moving away from engagementwith said extension when bowed, and springmeans urging said arm formovement when said extension is released.

' 22. A thermal relay comprising a straight bimetal strip, a supportingstructure therefor havin slotted end supports in the slots of which t eends of said strip are loosely received, means which restrains saidstrip against longitudinal displacement from said end supports, and apivotallysupported, spring-urged tripping arm disposed alongside saidstrip having an extension which underlies and engages a side edge ofsaid cool strip and is released from such engagement by the bowing ofthe heated strip.

In testimony whereof, I have signed my name to thisspecification.

GEORGE W. OKEEFFE.

